Since none of the previously entered salaries are needed to calculate ...

• the largest commission
• the total takehome

... you can skip the num[10] array.

• You do need to increase i in the loop. i+1 is an expression which does not change the value of i. I suggest using a for loop for this which keeps the declaration of i, the condition (i < 10) and the increment nicely in one place.
• You should check the return value from scanf. If you scan for one thing, it should return 1, otherwise it failed.
• Save the largest commission in the loop.
• Initialize variables.
• Add to totaltakehome. You currently assign a new value to it in every loop.

Example:

#include <stdio.h>

int main() {
    int largest = 0;       // initialized (and assuming this can't be negative)
    int totaltakehome = 0; // initialized

    for(int i = 0; i < 10; ++i) {  // loop with increment
        printf("Enter salesperson salary for week[%d]:", i + 1);

        int num;
        if(scanf("%d", &num) != 1) {   // check for success
            fprintf(stderr, "user error, aborting\n");
            return 1;
        }

        // an alternative commission forumla
        int com = num * 7 / 100; // perhaps a more fair commission

        printf("Commision is %d\n", com);

        // save this commission if it's the largest so far:
        if(com > largest) largest = com;

        // Add to `totaltakehome` - don't assign:
        totaltakehome += 300 + com;    // note: `+=`, not `=`

        printf("\n");
    }

    // print results after the loop:
    printf("Largest commision %d\n", largest);
    printf("Total takehome is %d\n", totaltakehome);
}